This invention relates to the configuration and manufacture of light-emitting devices suitable for use in flat-panel displays such as flat-panel cathode-ray tube (xe2x80x9cCRTxe2x80x9d) displays.
A flat-panel display CRT display typically consists of an electron-emitting device and an oppositely situated light-emitting device. The electron-emitting device, or cathode, contains electron-emissive elements that emit electrons across a relatively wide area. An anode in the light-emitting device attracts the electrons toward light-emissive regions distributed across a corresponding area in the light-emitting device. The anode can be located above or below the light-emissive regions. In either case, the light-emissive regions emit light upon being struck by the electrons to produce an image on the display""s viewing surface.
FIG. 1 presents a side cross section of part of a conventional flat-panel CRT display such as that described in U.S. Pat. No. 5,859,502 or U.S. Pat. No. 6,049,165. The display of FIG. 1 is formed with electron-emitting device 20 and light-emitting device 22. Electron-emitting device 20 contains backplate 24 and overlying electron-emissive regions 26. Electrons emitted by regions 26 travel toward light-emitting device 22 under control of electron-focusing system 28. Item 30 represents an electron trajectory.
Light-emitting device 22 contains faceplate 32 coupled to backplate 24 of electron-emitting device 20 through an outer wall (not shown) to form a sealed enclosure maintained at a high vacuum. Light-emissive regions 34 overlie faceplate 32 respectively opposite electron-emissive regions 26. When electrons emitted by regions 26 strike light-emissive regions 34, the light emitted by regions 34 produces the display""s image on the exterior surface (lower surface in FIG. 1) of light-emitting device 22. Contrast-enhancing black matrix 36 laterally surrounds light-emissive regions 34.
Light-emitting device 22 also contains light-reflective layer 38 situated over light-emissive regions 34 and black matrix 36. Regions 34 emit light in all directions when struck by electrons. Hence, some of the so-emitted light travels backward toward the interior of the display. Layer 38 reflects some of that rear-directed light forward to increase the intensity of the image. In addition, layer 38 functions as the display""s anode for attracting electrons toward light-emitting device 22.
The electrons emitted by regions 26 pass through light-reflective layer 38 before striking light-emissive regions 34. In so doing, the electrons lose some energy. The image intensity increase resulting from the light-reflective nature of layer 38 at least partially compensates for any image intensity decrease caused by this electron energy loss. Nonetheless, it would be desirable to further improve the image intensity in a light-emitting device whose anode overlies the device""s light-emitting regions.
Each light-emitting region in a light-emitting device such as that of FIG. 1 normally consists of light-emissive particles formed with phosphor material. The constituents of the phosphor particles commonly include elements such as sulfur or/and oxygen. When the light-emissive particles are struck by electrons, some of the sulfur or/and oxygen is commonly released in gaseous form into the interior of the display. The so-released gases can contaminate the display and cause it to degrade.
Petersen et al (xe2x80x9cPetersonxe2x80x9d), U.S. Pat. No. 5,844,361, addresses the problem of outgassing from phosphor particles in a light-emitting device of a flat-panel CRT display by chemically treating the outer particle surfaces in a way intended to reduce undesired outgassing. FIGS. 2 and 3 depict two examples of Petersen""s approach in which light-emissive regions overlie transparent substrate 40. Each light-emissive region consists of a layer of phosphor particles 42.
A coating 44 fully surrounds each phosphor particle 42 in the example of FIG. 2. Coatings 44 can alter the surface chemistry of particles 42 in such a way that they are more thermodynamically resistant to outgassing. Alternatively, coatings 44 can simply be impervious encapsulants that substantially prevent any contaminant gases produced by particles 42 from entering the display""s interior. In either case, coatings 44 are provided on particles 42 before they are deposited over substrate 40. The display""s anode is formed with aluminum layer 46 provided above composite particles 42/44.
In the example of FIG. 3, coatings 48 of stable oxide are provided on particles 42 after they are deposited on substrate 40. Each coating 48 conformally covers an upper portion of the outer surface of one particle 42. Coatings 48, typically formed by chemical vapor deposition of silane, disiloxane, or tetra-ethyl-orthosilicate, are more thermodynamically resistant to outgassing than are particles 42. Petersen indicates that the display""s anode in the example of FIG. 3 can be formed with a conductive layer analogous to aluminum layer 46.
Providing phosphor particles 42 with full coatings 44 before particles 42 are deposited on substrate 40 in the example of FIG. 2 raises concerns that coatings 44 may be damaged during the deposition of particles 42. Also, full coatings 44 may detrimentally affect the formation of the light-emissive regions by absorbing radiation typically utilized in defining the light-emissive regions. Petersen avoids this difficulty with the example of FIG. 3 where partial coatings 48 are deposited on particles 42 after they are deposited on substrate 40. However, Petersen only discloses that coatings 48 may consist of oxide. Petersen does not deal with improving the image intensity.
The present invention furnishes a light-emitting device in which a light-emissive region formed with a plurality of light-emissive particles overlies light-transmissive material of a plate. The light-emitting device of the invention is suitable for use in a flat-panel display, especially a flat-panel CRT display in which an electron-emitting device is situated opposite the light-emitting device. The electron-emitting device emits electrons which strike the light-emissive region, causing it to emit light.
The light-emissive particles in light-emissive region of the present light-emitting device are provided with coatings that perform various functions. In some cases, the particle coatings enable the intensity of light that travels generally in the forward direction to be enhanced, especially when the light-emitting device contains a light-reflective layer situated over the coatings. Alternatively or additionally, the particle coatings may cause the optical contrast to be enhanced between two such light-emissive regions when one of the light-emissive regions is turned on (emitting light) and the other is turned off (not emitting light). The coatings may getter contaminant gases. The coatings also typically reduce damaging effects that occur as the result of electrons striking the light-emissive particles.
Depending on the function or functions to be performed by the particle-coating material, each light-emissive particle may have two or more of the present coatings. In any event, each coating covers only part of the outer surface of the underlying particle in such a way as to be spaced apart from where that particle is closest to the plate. By configuring the coatings in this way, the coatings can be provided over the particles after they are provided over the plate, thereby avoiding difficulties that arise when light-emissive particles are provided with coatings before the particles are provided over a plate.
The light-emissive particles normally emit light in substantially all directions. Part of the emitted light travels generally forward, including partially sideways, toward the plate and passes through it. Part of the emitted light travels generally backward, likewise including partially sideways, away from the plate.
In a first aspect of the invention, each light-emissive particle is covered with a light-reflective coating positioned in the manner indicated above to conformally cover part of the particle""s outer surface. As a result, the particle coatings reflect forward some of the initially rear-directed light emitted by the particles. While the light-reflective layer normally situated over the particles above the light-reflective coatings performs generally the same function as the light-reflective particles, the combination of the light-reflective coatings and the light-reflective layer causes more light to be directed forward than would be achieved solely with the light-reflective layer. Hence, usage of the light-reflective coatings enables the light intensity to be increased in the forward direction.
The coatings are typically made light reflective by forming them from one or more of the metals beryllium, boron, magnesium, aluminum, chromium, manganese, iron, cobalt, nickel, copper, gallium, molybdenum, palladium, silver, indium, platinum, thallium, and lead, including alloys of one or more of these metals. Boron, aluminum, gallium, indium, and thallium, all of which fall into Group IIIB (13) of the Periodic Table, are attractive for the light-reflective coatings because none of these five metals is an electron donor. Silver and copper are attractive because they are substitutional species in metal sulfide phosphors suitable for implementing the light-emissive particles to respectively emit blue and green light.
In a second aspect of the invention, each light-emissive particle is partially covered in the preceding manner with a getter coating for sorbing (adsorbing or absorbing) contaminant gases. If the light-emissive particles produce contaminant gases as a result of being struck by electrons or/and other charged particles, the getter coatings can sorb the so-produced gases before they move away from the particles and cause damage elsewhere. When the light-reflective layer overlies the getter coatings, the light-reflective layer is normally perforated. Contaminant gases originating at locations away from the light-emissive region can thus pass through the light-reflective layer and be sorbed by the getter coatings.
The getter coatings are typically formed with one or more of the metals magnesium, chromium, manganese, iron, cobalt, nickel, copper, molybdenum, palladium, silver, platinum, and lead, including alloys of one or more of these metals. All twelve of these metals are particularly suitable for sorbing sulfur. Alternatively or additionally, the getter coatings can be formed with one or more of the metals titanium, vanadium, zirconium, niobium, barium, tantalum, tungsten, and thorium, including alloys of these additional eight metals. When the getter coatings are formed with one or more of the preceding twenty metals, the getter coatings may also be light-reflective for enhancing the light intensity in the forward direction as described above. Furthermore, the getter coatings can alternatively or additionally be formed with oxide of one or more of magnesium, chromium, manganese, cobalt, nickel, and lead, each of which is particularly suitable for sorbing sulfur.
In a third aspect of the invention, part of the outer surface of each light-emissive particle is conformally covered with multiple intensity-enhancement coatings. The number of intensity-enhancement coatings overlying each particle is, for convenience, designated here as plural integer m. The m coatings overlying each particle are similarly designated as the first coating through the mth coating, where the first coating is the nearest coating, i.e., the coating directly overlying the particle. Each ith coating overlies each (ixe2x88x921)th coating where i is an integer varying from 2 to m. Hence, the mth coating is the furthest, i.e., most remote, coating. A light-reflective layer normally overlies the intensity-enhancement coatings.
Each first coating is of lower average refractive index than the underlying particle. Each ith coating, where i again varies from 2 to m, is of lower refractive index than the (ixe2x88x921)th coating. In other words, the average refractive index decreases progressively in going from each particle to its nearest coating and then from its nearest coating to its furthest coating.
Light incident on an interface between a pair of light-transmissive media having different refractive indices is partially reflected at the interface and partially transmitted through the interface. With this in mind, the benefit of having the average refractive index decrease progressively in going from each particle to its nearest coating and then from its nearest coating to its furthest coating can be seen by considering the three-medium situation in which light travelling in a first medium is partially reflected and partially transmitted at an interface between the first medium and a second medium of lower refractive index, and the partially transmitted light travelling in the second medium is then partially reflected and partially transmitted at an interface between the second medium and a third medium of even lower refractive index.
The intensity of light reflection at an interface between two light-transmissive media varies with their refractive indices in such a way that, ignoring light absorption, the total fraction of light transmitted through both interfaces in the three-medium situation is greater than the fraction of light that would be transmitted through an interface between the two media having the highest and lowest refractive indices. In other words, placing a light-transmissive medium having an intermediate refractive index between two other light-transmissive media enables more light to be transmitted from the medium having the highest refractive index to the medium having the lowest refractive index than would occur if the media having the highest and lowest indices directly adjoined each other.
In view of the foregoing interface optics, arranging for the m coatings overlying each particle to have the above-described positional and refractive-index characteristics enables more light travelling backward and partially sideways to escape each particle and its coatings than would escape that particle in the absence of the coatings. Part of the light that escapes the particles travelling backward, including partially sideways, strikes the light-reflective layer in such a way as to be reflected generally forward to the sides of the particles. Accordingly, the intensity of emitted light is enhanced in the forward direction.
In a fourth aspect of the invention, part of the outer surface of each light-emissive particle is conformally covered with an intensity-enhancement coating of lower average refractive index than that particle. A contrast-enhancement layer, which appears dark as seen through the plate from opposite the light-emissive region, overlies the intensity enhancement coatings. The contrast-enhancement layer is typically divided into multiple contrast-enhancement coatings, each generally conformally overlying a corresponding one of the intensity-enhancement coatings. Once again, a light-reflective layer normally overlies the coatings.
The contrast-enhancement layer absorbs ambient light which impinges on the front of the light-emitting device and passes through the plate, the light-emissive particles, and the intensity-enhancement coatings. As a result, the contrast-enhancement layer improves the optical contrast between times when the light-emissive region is turned on and times when it is turned off. Hence, an improvement is achieved in the optical contrast between two such light-emissive regions during periods when one is turned on and the other is turned off.
The intensity-enhancement coatings in this aspect of the invention function generally the same as in the previous aspect of the invention to enable more backward-travelling light to escape the light-emissive particles and coatings than would escape the particles if the intensity-enhancement coatings were absent. Although the contrast-enhancement layer normally absorbs part of this backward-travelling light, the light-reflective layer reflects more backward-travelling light forward than would occur in the absence of intensity-enhancement coatings. The overall visibility of the image produced by multiple ones of the light-emissive regions is improved.
In a fifth aspect of the invention, each light-emissive particle is again partially covered with a conformal intensity-enhancement coating of lower average refractive index than that particle. A light-reflective coating similarly covers each intensity-enhancement coating. The intensity-enhancement coatings again enable more rear-directed light to escape the light-emissive particles and intensity-enhancement coatings than would escape the particles in the absence of the coatings. The light-reflective coatings reflect part of this increased amount of rear-directed light forward. When, as is typically the case, a light-reflective layer overlies the light-reflective coatings, the combination of the light-reflective coatings and the light-reflective layer enables more of the rear-directed light to be reflected forward than would be attained solely with the light-reflective layer. The light intensity in the forward direction is improved.
In a sixth aspect of the invention, part of the outer surface of each light-emissive particle is conformally covered with a contrast-enhancement coating without any intervening intensity-enhancement coating. The contrast-enhancement coatings appear dark as seen through the plate from opposite the light-emissive region. Each contrast-enhancement coating typically consists of multiple portions spaced apart from each other. Similar to the contrast-enhancement layer mentioned above, the contrast-enhancement coatings improve the optical contrast between times when the light-emissive region is turned on and when it is turned off. Consequently, the optical contrast is improved between two such light-emissive regions during periods when one is turned on and the other is turned off.
The particle coatings are located between the layer of light-emissive particles and the accompanying electron-emitting device in all six of the foregoing aspects of the invention. Although the coatings only partially cover the outer surfaces of the particles, the vast majority of the electrons emitted by the electron-emitting device strike the coatings before reaching the underlying light-emissive material of the particles. The particle coatings normally consist of material that does not become significantly volatile when struck by the electrons. Consequently, the particle coatings themselves normally do not pose significant contamination problems.
At the same time, the particle coatings reduce damaging effects, such as particle erosion and undesired outgassing, that arise when electrons strike the particles. Both performance and lifetime are improved. In fact, when the coatings contain one or more of the metals prescribed above for the light-reflective coatings in the first aspect of the invention, the preceding advantages can be achieved even though the coatings are insufficient, e.g., too thin, to provide significant light reflection.
Manufacture of a light-emitting device in accordance with the invention entails providing a layer of light-emissive particles over light-transmissive material of a plate to form a light-emissive region. The coatings are subsequently provided over the particles to provide one or more of the functions described above. When a light-reflective layer is to be included in the light-emitting device, the light-reflective layer is formed over the coatings.
In short, a light-emitting device configured and manufactured according to the invention has improved performance and increased lifetime. The present light-emitting device can readily be manufactured in a large scale production environment. By providing the particles with the present coatings after the particles have been provided over the plate, the invention avoids concerns, such as damaging the particle coatings, that can arise when pre-coated particles are deposited over a plate. Accordingly, the invention provides a substantial advance over the prior art.